APEX

Hey all... slowly making some progress with APEX. Taking quite a bit of getting used to...

Is there a functional equivalent to a Forms WHEN-LIST-CHANGED trigger?

What I need to accomplish: I have a Select List field. Based on the value of that item, the value of two separate Radio Group items needs to be set to values returned by a query. In Forms, WHEN-LIST-CHANGED makes this very easy, but I can't seem to find a way to make something work at the same "trigger point" in APEX. I've tried a validation with a "Value of Item in Expression 1 Is Not Null" with :[item name] in the "Expression 1" box, but it doesn't seem to work...

What am I missing here?

Thanks for any help...